Add scanning of multiple targets.(+ Minor improvements)

[leddcode]

What does it implement/fix? Explain your changes.

Current PR adds an option for scanning multiple targets from a list in a simple manner.

I have added the -ul flag, which expects a path to a file with targets.
Usage: python xsstrike.py -ul path/to/urls.txt --skip
A target file may contain urls from a variety of sources:
https://example1.com/?q=aaa
https://example2com/?q=aaa
https://example3.com/?q=aaa
etc..

The tool displays a summary of vulnerable targets after iterating all targets.

Where has this been tested?

Python Version: 3.9
Operating System: Windows 10

Does this close any currently open issues?

#353

Does this add any new dependency?

No

Does this add any new command line switch/option?

Yes, -ul flag to provide a path to a file with targets.

Any other comments you would like to make?

Great tool! Thanks for all contributors.

Some Questions

• [ *] I have documented my code. - The code is very simple and doesn't require to be documented.
• [V ] I have tested my build before submitting the pull request. - Sure!

[ItsIgnacioPortal]

Since @s0md3v seems to have abandoned this project, I've gone ahead and took the liberty to make a fork: XSStrike-Reborn
Your pull request has been merged into that project. Your efforts are appreciated 😁

[leddcode]

@ItsIgnacioPortal This is an excellent initiative! Thanks a lot, and well done for taking things forward!